Work in this laboratory has been directed toward the analysis of a particular anti-allotype antisera. This reagent and the antigenic determent it recognizes (A34) possess a number of interesting attributes. The antisera appears to react with the VH region of some Ig molecules in essentially all (50) rabbits tested (by RIA) including preimmune sera of the immunized rabbit. Thus, this is an auto-anti-allotype reagent. Normal rabbits only have between .01 and 10 microgram/mg of Ig bearing the A34 allotype. In contrast, rabbits which are VH allotype (a) suppressed, express from 100 to 300 times the control level of this Ig. Serological analysis indicates that there are two distinct patterns of reactivity, each of which are linked to one or another of the known VH allotype subgroups, thus, allelic forms of the determinant appear to be expressed. The low level of this allotype suggest that a continued search for additional minor allotypes may be fruitful. A second area of study concerns the expression of the so-called a-allotypes in normal specifically induced antibody populations. An a-allotype (y33) was found to be expressed in low levels (0.1-5.5 percent of total Ig) in normal individuals as expected. Specific antibody from rabbits immunized to Micrococcus lysodeikticas or the b4 allotype of rabbit immunoglobulin were also found to express low levels (0.1 to 12 percent) of y33. As the immune response progressed, the proportion of y33 Ig in the total Ab population remained low or decreased (from 12 percent to 3 percent in one animal). This indicates a general non competitiveness of the a-allotypes when a plus allotypes are expressed.